Combustion device

ABSTRACT

A combustor ( 1 ) has a swirler ( 22 ). The swirler has mixing channels ( 40 ), the mixing channels having a height/width aspect ratio of less than ( 2 ). A secondary fuel inlet ( 32 ) is located downstream of mixing channel ( 40 ) and may be in a zone ( 65 ) of separated flow. Primary fuel admission may be through a threaded or push fit circular movable plug ( 100 ). The plug ( 100 ) may be conveniently removed for calibration purposes and may have a bell-mouthed entrance.

BACKGROUND OF THE INVENTION

The present invention relates to a combustion device, and moreparticularly but not exclusively to a combustion device for a gasturbine engine, and furthermore to the assembly of components forming acombustion device. A combustor is commonly used in a gas turbine engineto burn fuel in compressed air to produce exhaust gas for exhaust to aturbine. Recent combustor designs have aimed at reducing emissions ofnitrogen oxide (NO) and nitrogen dioxide (NO₂)— collectively known asNOx. Running the combustor to produce well-mixed fuel and air in amixing channel can decrease NOx emissions. However, this level of mixingcan create flame instability at low flow rates, leading to flameblow-out. Consequently, it is known to use a secondary or pilot fuelinlet to prevent the blow-out from occurring. One combustor design has astub tube having several fuel inlets arranged to admit fuel into amixing channel to increase mixing and reduce NOx emissions. However,this arrangement is prone to flame instability and potential flameblow-out. A stub tube creates a wake, which reduces the effective areaof the mixing channel and reduces the flow rate of air and fuel throughthe mixing channel. Each of these multiple inlet designs has the problemthat each inlet must be calibrated at every position individually. Thisis time consuming and fiddly, and can also result in costly downtime. Anobject of the present invention is to relieve the problems of the priorart in a simple and effective manner at no major expense.

Furthermore, radial swirlers for industrial gas turbine combustorstypically involve a series of rectangular mixing ducts issuingtangentially into a closed end cylindrical chamber. This arrangementresults in the formation of a stable solid body rotation of sufficientstrength for the formation of a recirculation zone to provide for flamestability. The highly swirling flow within the chamber also provides foradditional mixing. The use of rectangular ducts does, however, haveseveral disadvantages. The need to achieve a uniform fuel distributionwithin each duct for low emission performance can necessitate a complexinjection arrangement, such as fuel rods projecting across each duct.The inter duct discharge coefficient may also show some variation, dueto slight variations in duct aspect ratio, impacting on mixtureuniformity within the combustion chamber. Finally, the use ofrectangular ducts dictates certain manufacturing methods which may notbe conducive with low cost or ease of production.

Various aspects of the present invention are set out in the independentclaims. A number of optional features are set out in the dependentclaims. The optional features are applicable to each aspect of theinvention and the invention envisages and extends to any combination ofthe aspects and optional features hereof which is not specificallyrecited herein.

SUMMARY OF THE INVENTION

When an insert channel portion/plug is provided, this may be removablyattached to the body, such as by threaded engagement or a push fit, orpermanently attached, such as by braising.

According to another aspect of the present invention, there is providedan apparatus for mixing compressed air with fuel, comprising a bodyhaving a mixing channel for mixing fuel and air, a primary fuel inletand a secondary fuel inlet, wherein the secondary fuel inlet is adaptedto admit fuel into a zone of separated flow on the body. In a furtheraspect of the present invention, the secondary fuel inlet is positionedoutside of the mixing channel. The advantage of this is that arelatively small amount of fuel may be admitted to a zone in which thereis little mixing of air and fuel, therefore providing flame stabilityand avoiding flame blow-out, whilst the majority of fuel is admittedthrough a primary fuel inlet in the mixing channel to produce a wellmixed mixture of air and fuel.

According to another aspect of the invention, the mixing channel has arectangular cross section, having a width dimension defined in the axissubstantially parallel to the plane of the swirler body, and a depthdimension defined in the access substantially orthogonal to the plane ofthe body, wherein the mixing channel aspect ratio is such that itsdepth-to-width ratio is less than or equal to 2. In yet another aspectof the invention, the same depth-to-width is less than or equal to 1.5,preferably ≦1.25 or ≦1.0. This ratio is preferably ≧0.7. The aspectratio may be ≦2, ≦1.5, ≦1.2, ≧0.5 and/or ≧0.7. This ratio may be from0.5 and 2, preferably from 0.7 to 1.5, e.g., from 0.8 to 1.2, oneexample being 1.0. Accordingly, better mixing of the air and fuel isachieved than would be obtained by a taller mixing channel having ahigher depth-to-width ratio.

According to a further aspect of the invention, the apparatus has a fuelmetering means having an elliptic or otherwise curved or partly curvedcross section such as circular, oval or racetrack-shaped. In anotheraspect of the invention, the fuel metering means is removable and mayalso be of elliptic cross section.

A number of further optional features, which may be applicable to anyone or more of the above aspects of the invention, will now bedescribed.

Preferably, the mixing channel comprises a bore formed in the body ofthe apparatus, the bore having an elliptical cross section, which may becircular. The advantage of an elliptic cross section channel is thatfuel may be conveniently admitted into the channel from around thechannel, so as to increase mixing of the fuel with air entering thechannel, and so the channel flow does not suffer from the reduced effectof area that can occur in the corners of rectangular channel flows.

Preferably, the zone of separated flow is outside of the mixing channelso as to avoid reducing the mixing channel effective area. The secondaryfuel inlet is more preferably positioned downstream of the mixingchannel. The advantage of this is that air and fuel are well mixed priorto secondary fuel being added, thus minimising NOx emissions, and yetthe pilot (or secondary) fuel is available when the mixture isvulnerable to flame blow-out. The body may be a swirler. The mixingchannels may be equi-spaced around the circumference of the swirler, andthe secondary fuel inlets may be equi-spaced around the center of theswirler to facilitate good mixing of air and fuel. In other cases, thesecondary fuel inlets may not be equally spaced. Preferably, thesecondary fuel inlets are positioned on axes, each axis being alignedwith the longitudinal axis of a mixing channel. Hence, fuel may beadmitted into multiple streams of air and the mixture is able to enterthe air/fuel chamber from several directions simultaneously, creating acircumferentially uniform influx of the mixture into the air/fuelchamber. In one aspect of the invention, there may be an equal number ofsecondary fuel inlets and primary fuel inlets, and in another aspect ofthe invention, there may be fewer secondary fuel inlets than there areprimary fuel inlets. Preferably, the mixing channels are oriented so asto impart a swirl component of motion to the air/fuel mixture exitingthe mixing channels, such that a vortex is formed in the air/fuelchamber producing a low pressure core in the air/fuel chamber flow. Thelow-pressure core will induce mixture in the air/fuel chamber torecirculate back up the chamber such that any excess fuel in the mixturecan be burnt.

The removable means for metering of the fuel preferably comprises afuel-metering insert. An advantage of a removable fuel metering means isthat the means may be calibrated outside of the apparatus and theninstalled on the apparatus. This is not only more convenient for thecalibrator, but also avoids having to stop the apparatus in order toperform the task, reducing costly downtime. Preferably, thefuel-metering insert comprises a bell mouth entrance and a main insertbore, the bell mouth reducing pressure losses as intake air enters theinsert. Preferably, the fuel-metering insert is insertable into themixing channel insert bore and may be threadably insertable (or a pushfit) into the mixing channel insert bore for ease of installation andremoval thereof.

When installed onto the apparatus, the insert's bell-mouth entrance maydefine the mixing channel entrance. Preferably, when the insert isinstalled on the apparatus, the mixing channel then comprises the bellmouth entrance through which mixing air enters the insert, the maininsert bore, the body channel main bore and exit. Preferably, the mixingchannel body bore has a cross section graduating from being ellipticaladjacent the mixing channel insert bore to rectangular at the exit, suchthat the flow exiting the mixing channel is tangential to the body,producing a uniform vortex in the air/fuel chamber. The insert's mainbore may comprise an elliptical cross section having a similar crosssection to that of the mixing channel body bore, thus ensuring a smoothtransition between the two bore sections. This will allow the air/fuelmixture to flow undisturbed to the channel exit. The elliptical crosssection is preferably a circular cross section to obtain the advantagealready mentioned.

The insert may comprise an opening for the admission of fuel into themain bore of the insert, and the opening may be in fluid communicationwith a primary fuel manifold. There is preferably a plurality ofopenings spaced around the perimeter of the fuel-metering insert tofacilitate good mixing in the mixing channel. The fuel-metering insertmay include an annular manifold in fluid communication with the primaryfuel inlet at one end thereof and with the primary fuel manifold at theother end. The opening may be the primary fuel inlet. Preferably, thefuel metering means is positioned so that the insert bore is tiltedupwards towards the center of the body.

The air/fuel mixing apparatus is preferably a combustor, which may havea body, an air/fuel chamber, a casing and an exhaust. The casing, bodyand air/fuel chamber may be adapted so that intake air, preferably froma compressor, flows through the gap between the casing and the air/fuelchamber prior to entering the mixing channel. The body may be a swirleradapted to induce a low-pressure core in the air/fuel chamber. Thelow-pressure core caused by the swirler may induce a recirculation zonein the air/fuel chamber. The swirler may include a surface in fluidcommunication with the air/fuel chamber, the surface preferably having aboundary layer adjacent to it that includes an attached portion and aseparated zone downstream of the attached portion. The swirler maycomprise a fuel manifold having a primary fuel manifold and a secondaryfuel manifold for admission of fuel into the swirler. The fuel may be afluid, and is more preferably a gas.

According to another aspect of the invention, a method of mixing air andfuel comprises the admitting of fuel through a primary inlet and asecondary inlet into a body, wherein the fuel admitted by a secondaryfuel inlet is admitted to a flow separation zone on the body.

According to an alternative aspect of the invention, a method ofcalibrating a fuel metering means comprises calibration of the fuelmetering means and then installation of the fuel metering means on anapparatus for mixing air and fuel.

In a preferred construction, an improved method of fuel injection for alean, well-mixed system for use in a low NOx combustor is provided incombination with a radial inflow swirler/mixing apparatus, employed forestablishing a flame stabilizing zone. A preferably movable fuelinjector or plug gives advantages over conventional methods in terms ofmixing length, flow prediction, ease of calibration, clean aerodynamicsand pressure loss. The injector/removable plug/portion of the mixingapparatus is essentially a bell-mouthed air metering orifice of highdischarge coefficient, which preferably screws or push-fits into thebody of the swirler/mixing apparatus which is preferably of the radialinflow type. The injector is preferably surrounded by a gas fuel galleryfrom which the fuel is injected into the mixing channel through one ormore fuel metering holes. The size and number of the holes may beselected to control the quantity of fuel injected. Mixing of fuel andair may be achieved in a relatively short distance from the point ofinjection. In contrast, other conventional injection systems arebelieved to incorporate stub tubes and wall injectors needing longpassage lengths to achieve low standard deviation of mixing and low NOxproduction in the subsequent flame zone. In addition, these knowninjectors often upset the aerodynamics and discharge coefficient of themixing channel, calling for individual calibration and higher pressurelosses. Additional benefits from improved injection/mixing of fuel maybe the option of selecting a lower combustion zone airflow and thereforewider flame stability margin at a given level of NOx production and thebenefit of such a change may be the allocation of more air for coolingother parts of a combustor on which the mixing apparatus may beemployed.

The mixing apparatus may be employed for gaseous fuel althoughapplications for liquid fuels are also envisaged.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be explained in more detail by thefollowing non-limiting description of preferred embodiments and withreference to the accompanying drawings, in which:

FIG. 1 is a view through a section of a combustor apparatus according toa first preferred embodiment of the invention;

FIG. 2 is a detail of a portion of the apparatus of FIG. 1;

FIG. 3 is a plan view of a swirler of the apparatus of FIG. 1;

FIG. 4 is a sectional elevation of a combustor apparatus according to asecond preferred embodiment of the invention;

FIG. 5 is a detail of a portion of the apparatus of FIG. 5;

FIG. 6 is a plan view of a swirler of the apparatus of FIGS. 4 and 5;

FIG. 7A is a cross section of a mixing channel of the apparatus of FIG.2;

FIG. 7B is a cross section of a mixing channel according to a modifiedembodiment of the invention;

FIG. 8 is a sectional elevation of a combustor apparatus according toanother embodiment of the invention;

FIG. 9 is a detail of parts of the apparatus of FIG. 8;

FIG. 10 shows a sectioned plan view and a side view of a swirler of theapparatus of FIG. 8;

FIG. 11 shows a fuel metering means of the apparatus of FIG. 8.

FIG. 12 shows a partial perspective cross section through the swirlerplate of the embodiment of FIGS. 8 to 11;

FIG. 13 is a schematic view showing the various combustors in accordancewith the embodiments of the present invention which may be incorporatedin a gas turbine engine;

FIG. 14 shows a modified secondary/pilot inlet system, having an annularrecess and ring;

FIG. 15 is a cross section through mixing channels of a swirler plate ofa further embodiment of a combustor in accordance with the invention;

FIG. 16 shows the swirler plate of FIG. 15 secured on a combustion linerand with back plates for secondary fuel inlets and an ignitor securedthereto;

FIGS. 17A to 17D show a modification of the embodiment of FIG. 1 inwhich secondary/pilot fuel inlets are shielded;

FIGS. 18A to 18H show a different modification to the embodiment of FIG.1 in which a radial pilot/secondary fuel arrangement is provided; and

FIGS. 19A to 19C show a modification to the embodiment of FIG. 16 inwhich fully circular mixing ducts are used issuing tangentially into atoroidal chamber.

FIG. 1 shows a sectional elevation through a combustor in accordancewith the invention. The combustor 1 comprises a casing 2, a liner (orcombustion chamber) 4, a fuel manifold 6 and an exhaust 8. The casingconsists of an outer casing 10 having an internal insulation 12, and atop flange 14. The casing 10 is generally cylindrical with a rectangularshape in side section. The combustion chamber 4 comprises a pre-chamber16 expanding to the main chamber 18. At an end of the combustion chamberopposite the pre-chamber 16 is a contraction 20 that narrows to anexhaust 8. The fuel manifold 6 comprises a swirler 22, fixedly attachedto the top flange 14 by one or more clamping studs 24 as shown in FIG.2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 and FIG. 3 show the swirler 22. In the embodiment described, theswirler 22 consists of a circular plate 23 although other shapes may beused. FIG. 3 shows a plan view of the swirler 22. The swirler comprisesan outer annulus 34 inside of which is located generally triangularwedge sections 36. The wedge sections 36 are, in the present embodiment,of a generally wedged shape, having a longer side 42 and two shortersides 38 and 44 in which a shorter side 38 is curved to the samecurvature as an inner circumference 35 of the annulus 34. The wedgesections 36 are fixedly attached to the annulus 34 at the curved side38, and arranged inside the annulus 34 so as to form channels 40 betweenthe straight longer side 42 of one wedge 36 and the straight shorterside 44 of another. The channels 40 are thus inclined at an angle to theswirler radius such that the longitudinal axis of a channel 40 does notpass through the origin of the circular swirler 22. The channels 40 maybe substantially straight or they may be curved. The channels 40 exitinto the pre-chamber 16. Ignitor 31 is positioned off-center as in FIG.8, although it may appear central when viewed from certain directionse.g., FIG. 4. The ignitor 31 is off-center since this will be coolerthan a central location.

A primary fuel inlet 46 for admission of fuel into a channel 40 ispositioned upstream of the channel 40 although it may be positionedinside the channel, preferably towards the annulus than towards thechannel exit 48 as defined by the dashed circular line in FIG. 3. Asecondary fuel inlet 50 for further admission of fuel is located outsideof the channel just beyond the exit 48. In this embodiment, there is asecondary fuel inlet for every primary fuel inlet. The primary fuelinlets 46 and secondary fuel inlets 50 are located along thelongitudinal axes of the channels 40 although they could be positionedoff axis. FIG. 1 shows a primary fuel connection 30 and a secondary fuelconnection 32 that are adapted to admit fuel into the swirler 22. FromFIG. 2, it is seen that the primary fuel connection 30 is located withinthe top flange 14 such that it may be accessed at one end 31. Its otherend is in fluid communication with a primary fuel distributor 52, whichin turn is also in fluid communication with the primary fuel inlet 46. Aplurality of fuel distributor seals 54 surround the primary fueldistributor 52. The primary fuel connection 30, distributor 52 and inlet46 are aligned with each other in the present embodiment and arepositioned upstream of the channel 40.

A secondary fuel connection 32 is located within the top flange 14 suchthat it may be accessed at one end 33. Its other end 37 is in fluidcommunication with the secondary fuel distributor 58 which in turn isalso in fluid communication with the secondary fuel inlet 50. Aplurality of fuel distributor seals 54 surround the secondary fueldistributor 58. A secondary fuel connection 32, distributor 58 and inlet50 are aligned with each other in the present embodiment and arepositioned downstream of the channel exit 48.

The wedge sections 36 each have a through-bore 29 extending through theplate such that clamping studs 24 may extend therethrough. Each clampingstud 24 holds together the swirler 22, top flange 14 and a flange 60 ofthe air/fuel chamber or liner 4. Each clamping stud is inserted througha bore 61 in the flange 60, then into the swirler bore 29 before it isinserted into the top flange 14. A nut 62 locks the clamping stud inplace.

The mixing channel cross section may be as in FIG. 7A or FIG. 7B. Theratio of mixing channel height L along the combustor axis to width W mayin preferred embodiments be such that L/W is more than or equal to 0.7and less than or equal to 2.

According to a second embodiment of the invention, as shown in FIGS. 4to 6, the secondary fuel inlets 50 are positioned further downstream ofthe channel 40 and there are fewer secondary fuel inlets 50 than thereare primary fuel inlets 46. The secondary fuel inlets are placed so asto admit fuel into a zone of separated flow 65 at the swirler wall 64adjacent the pre-chamber 16. The secondary fuel connection 32 andsecondary fuel distributor 58 are also positioned in alignment with thesecondary fuel inlets 50. Thus the secondary fuel inlets are positionedcloser to the point of ignition 31 as shown in FIG. 4.

During operation of the gas turbine engine, compressed air enters thecombustor 1 through a gap 11 between the internal insulation 12 of thecasing 10 and the liner or combustion chamber 4. The mixing airflow pathas shown by the arrows in FIG. 1 and FIG. 4 passes through the gaptowards the top flange 14, near which it then enters the channel 40.Fuel is added to the air via primary fuel inlet 46 in a predeterminedand precalibrated amount so as to produce a well-mixed mixture of airand fuel in the mixing channel 40. A boundary layer 13 forms at themixing channel walls (FIG. 7A). The air/fuel mixture flows through thechannel exit and enters the pre-chamber 16. Upon immediately exiting thechannel 40, the boundary layer remains attached to a swirler/pre-chambersurface (or back plate) 64. The orientation of the channels 40 imparts aswirl motion to the mixture, causing the flow exiting the channels toform a vortex as it is drawn into the pre-chamber 16. As the vortex isformed, the boundary layer at the surface 64 separates, creating aseparation zone 65 on the surface 64. The secondary fuel inlet 50 isplaced outside of the channel 40 so as to provide a pilot to avoid flameblow-out. In some embodiments of the invention, the secondary fuel inlet50 is placed in the separation zone 65. It is believed that littlemixing of air and fuel takes place in the separation zone, hence thezone may have a stable flame that is far less likely to blow out thanthe flame of the well mixed air/fuel mixture. The introduction ofsecondary fuel at this region 65 also acts as a pilot to reduce thechance of flame blow-out elsewhere in the pre-chamber.

As the vortex forms in the liner (or combustion chamber) 4, alow-pressure region is formed at the vortex core. A portion of the flowat the vortex is induced into the core such that flow reverses directionat the central axis of the vortex near to the swirler. Thus, any excessfuel that was not burnt at the separation zone 65 is returned to theflame front at the point of ignition to be burnt. The remaining air/fuelmixture flows through the liner or combustion chamber 4 in the vortexand is exhausted to a turbine.

The channels 40 may be of rectangular cross section and may have anaspect ratio as shown in FIG. 7A, in which the channel height (L) 66 towidth (W) 68 ratio is 1.25 to 1. However, in another embodiment of theinvention, the channel height 66′ to width 68′ ratio is less than orequal to 1.0 as shown in FIG. 7B. FIGS. 7A and 7B are views along thelongitudinal axis of the mixing channel. These low aspect ratios enablegood mixing since fuel may be injected substantially right across thechannel to the wall opposite inlet 46.

A further embodiment of the invention is shown in FIGS. 8 to 11. FIG. 11shows a schematic of a fuel-metering insert 100. The insert 100comprises an annular plug of elliptical cross section, and preferably ofcircular cross section. The insert 100 has a bell-mouth flange 110 atone end thereof. The flange protrudes outwardly away from thelongitudinal axis 112 and defines a bell-mouth entrance 113. The borewall 114 (FIG. 9) has a threaded section 118 around its peripheralsurface 120. In the remaining section of the bore wall 114 is an opening122 (FIG. 11) providing a through-bore from the inner wall 117 to theouter wall 120 of the bore wall 114. In the present embodiment there arefour such openings. The fuel-metering insert 100 is insertable in andremovable from the air/fuel mixing apparatus 1. The channel 40 includesan inlet 125. In this embodiment of the invention, the channel 40includes a portion 130 at the inlet end thereof, which is threadedaccording to the opposite of the insert threaded portion 118. Thefuel-metering insert 100 is threadably insertable into the channel inlet125. The channel 40 is profiled substantially according to the female ofthe male insert outer surface 120 profile such that a close fit betweenthe two components is ensured upon threadably inserting the insert 100into the channel 40 of the air/fuel mixing apparatus 1. The insert isremoved by unscrewing the insert from the channel 40. The insertopenings 122 may thus be calibrated for fuel admission in a convenientmanner whilst the insert is removed from the apparatus. Whilst it isremoved, another calibrated insert may replace it, allowing thecontinued use of the apparatus and minimizing the downtime during whichthe apparatus cannot be used.

The fuel metering insert 100, through-bore 116 and the channel 40 haveidentical cross sections at the junction between them, and onceinstalled on the apparatus, the bore 116 is in exact fluid communicationwith the channel 40. Thus, the channel section downstream of the insertin this embodiment of the invention is elliptical, preferably circularin accordance with the cross section of the insert through-bore 116. Thechannel 40 may be elliptical at the junction with the inserts 100 andgraduates to a generally rectangular cross section at the channel exit48.

The openings 122 are, at the insert outer surface 120, in fluidcommunication with an annular fuel manifold 125, which is in turn intofluid communication with the primary fuel distributor 52. Manifoldsealing ring 54 surrounds the primary fuel distributor 52. Manifold 125is shown in further detail in FIG. 12

The installed insert is tilted upwards towards the fuel manifold, suchthat the channel 40 subsequently bends to be integral with the generallyhorizontal plane of the swirler upstream of the channel exits 48.

In use, gas fuel is administered into the air/fuel mixing apparatusthrough the primary and secondary fuel connections 30,32. Fuel from theprimary fuel connection passes through the primary fuel distributor 52from which it enters the annular fuel manifold 125. Fuel spreadsthroughout the annular manifold 125 and enters the insert through-bore116 at the openings 122. Inner insert seal 121 and outer insert seal 123ensure that fuel does not escape from the insert other than via openings122. Meanwhile, air from the compressor flows through the gap betweenthe casing 10 and the air/fuel chamber 4 and enters the insertbell-mouth entrance 113 where it passes through the bell-mouth and intothe through-bore. Mixing of air and fuel occurs in the through-bore 116and channels 40 before the mixture exits the channels. Secondary fuelmay be added, as with embodiments 1 and 2, before the mixture enters thepre-chamber in the same manner as therein described. In the secondarymanifold, it is preferred that the majority of fuel entering the swirler22 is admitted via primary fuel inlet 46 and that a much smaller amountof fuel is admitted via secondary fuel inlet 50. As shown for example inFIG. 9, the secondary inlet 50 is configured at an angle to inject fuelwith a component along the back wall 150 of the swirler. This componentis aligned with a corresponding mixing channel's direction and centralaxis. This allows the secondary or pilot fuel to remain near the backwall as a source of rich mixture which assists in preventing unwantedflame out. As shown by dotted lines in FIGS. 9 and 10A, one or more ofthe secondary inlets 50 may (in addition to or as an alternative tohaving the angled configuration) be provided by a shield 152 having anoutlet 154 facing away from a co-operating mixing channel 40, the shield152 thus conforming to an exit direction of the channel 40. The shieldserves to improve resistance to unwanted flame out and is useful for hotstarts. The shield may be welded in position or cast as part of the backwall 260. In one embodiment, four shielded pilots (secondary inlets 50)are used out of a total of eight pilots. As a further alternative, theback wall 260 may be provided with a recessed ring for all pilots, e.g.,all eight pilots when eight are used, as indicated schematically in FIG.14 in which an annular ring 160 coaxial with the swirler central axis162 is fitted in the region of eight secondary, pilot inlets which arelocated in an annular recess 164. This provides a radially inward flowof pilot fuel along the back wall 260.

The fuel is a fluid, most preferably a gas such as propane or naturalgas. However, it may be a liquid such as diesel.

FIG. 13 shows schematically the configuration of the combustor 1 in agas turbine engine 200′ having a main air compressor 202′ connected to ashaft 204′ to a turbine 206′ and an alternator 208′. A gas boostcompressor 210′ is provided between gas fuel 212′ and the combustor 1.The compressor 202′ is fed by an air inlet 214′, and the turbineexhausts and exhaust conduit 216′. The gas turbine engine may berecuperated in a known way.

FIGS. 15 and 16 show a revised version of the embodiment of FIGS. 8 to12. As shown by the cross-sectional view of FIG. 15, eight mixingchannels 40 are shown in a swirler casting 200. The mixing panels eachhave an upstream circular section 202 merging into a rectangulardownstream section 204. Inserted into the entrance 206 of each mixingchannel 40 is a bell-mouthed circular insert 208 having four primaryinlets for fuel 210 spaced equally around the inside thereof, three ofwhich are shown in the cross-sectional view of FIG. 16. The primaryinlets 210 of the eight mixing channels 40 are connected via manifoldchannels 212, cross sections of which are shown in FIG. 15, the manifoldchannels 212 being supplied by a supply port 214, shown schematically inFIG. 16. A secondary or pilot fuel supply plate 216 and a further backplate 218 are fixed to the swirler casting 200 by bolts 220. One ofseveral secondary fuel inlets 222 is shown schematically in FIG. 16,this secondary fuel inlet having an angled injection conduit 224 likethe one shown in FIG. 9. In addition to or alternatively to this, one ormore secondary inlets may be provided with a shield or may be located inan annular ring-type shield. An off-center ignitor 228 is secured inposition on the back plate 218. The supply plate 216 includes a fuelsupply gallery/manifold (not shown) for the various secondary/pilot fuelinlets 222. The swirler casting 200, back plate 216 and back plate 218effectively replace the top flange 14 and swirler 22 shown in FIG. 8 andFIG. 16 shows how these components are fitted on the liner 4. In theembodiment of FIGS. 15 and 16, the mixing channels 40 and thebell-mouthed circular inserts 208 are arranged perpendicular to thecentral axis of the swirler casting 200 and this can be contrasted withthe arrangement in FIG. 8 in which the mixing channel inlets are tilted.However, in other embodiments, it is envisaged that a cast swirler platelike the one shown in FIGS. 15 and 16 could have tilted mixing channelinlets.

The secondary inlets may be arranged for proportionate or on/off flow.In one embodiment, when secondary inlets are on, 70% of flow may bethrough primary inlets and 30% through secondary inlets for pilot fuel,although this may be variable and may be different in other embodimentssuch as a 60:40 split.

As shown in FIG. 17A to 17E, the swirler 22 may be replaced with aswirler 322 in which each or some of the secondary fuel inlets areshielded by shields 300, perspective and cross-sectional views of eachshield 300 being shown in FIGS. 17D and 17E respectively. In theembodiment shown, eight secondary/pilot fuel inlets 350 are provided,with four of the inlets 350 being shielded by shields 300. Each shieldis aligned with an axis of a corresponding mixing channel 340 with ashield fuel exit aperture facing away from the direction of incomingflow through the corresponding mixing channel 340.

FIGS. 18A to 18H show a different modification to the swirler 22 ofFIG. 1. In this case, the swirler 422 is modified as shown in FIGS. 18Gand 18H by providing a deflector plate 424 over a series of eight pilotoutlets 450. Eight pilot outlets 450 are provided. The deflector plate424 is circular as shown in FIGS. 18E and 18D. The plate 424 is providedwith a slight lip 426 which provides a gap between outlets 450 and arear face 428 of the plate 424. The deflector plate forces pilot flowradially inwards towards the center of the swirler 422. A possibleextension to this arrangement is to provide one or more relief holes inthe plate 424 so that pilot flow is split between radial and axiallyfuel flow to achieve best performance. In this case, a small relief holemay be drilled or otherwise provided through the plate 424 at eachlocation axial aligned with or near to one or more or all of thesecondary/pilot fuel outlets 450. The shields 300 or deflector plate 424provide improved performance and a stable flame.

FIGS. 19A to 19C show a modification to the embodiment of FIG. 16 inwhich completely circularly cylindrical mixing channels 500 areprovided. The circular mixing channels or premixing ducts 500 exittangentially into a toroidal space 502. The mixing channels 500 canreadily accommodate bell mouth shaped entrances 504 like the bell mouthshown in FIG. 16. For clarity, the bell mouth inserts are shown removedfrom FIG. 19A but in practice would be similar to those shown in FIG.16. The schematic views of FIGS. 19D and 19C show bell mouths 504.Curved lines 506 in FIG. 19B indicate the line of contact 508 betweencircular mixing channels 500 and toroidal surface 510, lines 507schematically providing a similar impression in side elevation in FIG.19C. Circular ducts 500 can be manufactured by simple drilling andreaming operations. Fuel placement is easier in a circular duct than arectangular duct due to the absence of corners where excessive fuel canget trapped. The toroidal geometry provided by the surface 510 providesa smooth transition from circular premix channels 500 into toroidalchamber 510. The swirling flow generated in the toroidal chamber 510 isthen accelerated into a cylindrical pre-chamber 516. The toroidalchamber prevents significant flow separations and recirculations at theduct/chamber interface and therefore prevents unsteadiness in the bulkswirling flow within the chamber 510/516 and unsteadiness leading to thegeneration of unacceptable combustion oscillations and possibleflashbacks, as well as the presence dead-zone-induced auto-ignition asfuel trapped within such regions may experience excessively longresidence times.

The swirler block 522 incorporates eight channels 500, although 10 or 12may also be used or other numbers if desired. Each duct issuestangentially into the toroidal shaped chamber 510 which essentially haspart-toroidal side surfaces 512 a flat back surface 514 and an open exit518 leading into the chamber 516. Each premixing channel 500incorporates a bell mouth 504 so that a repeatable discharge coefficientcan be achieved across all of the channels 500 to ensure an even flowdistribution. Located in each bell mouth are either three or fourequi-spaced fuel injection points to provide for a uniform injection offuel into the premix duct, although the number of injection points maybe varied. The injection points are preferably equi-spaced peripherallyaround the bell mouth, as in the embodiment of FIG. 16. The height ofthe toroidal chamber is set to be equal to the diameter of the premixducts so that a smooth transition from duct 500 to chamber 510 isachieved without flow separation. The arrangement of tangential ductsand toroid chamber results in the formation of a stable swirling motionwithin the chamber 510. The swirling motion is sufficiently strong thata recirculation zone is established within the combustor to provide astabilizing mechanism for the flame. The swirling flow accelerates outof the toroidal chamber 510 into the cylindrical pre-chamber 516 priorto issuing into the main combustor. Accordingly, it will be appreciatedthat the swirler plate 522 may essentially replace the equivalent plateshown in FIG. 16 in the overall combustor arrangement. A shielded orradial deflector arrangement may be used as desired with the embodimentshown in FIGS. 19A to 19C.

Various modifications may be made to the embodiments described withoutdeparting from the scope of the invention as defined by the followingclaims, as interpreted under patent law.

1-78. (canceled)
 79. A mixing apparatus for mixing fuel and air forcombustion in a gas turbine, the mixing apparatus comprising: a bodyhaving a mixing channel for mixing fuel and air for combustion, themixing channel having a main channel portion and a distinct insertchannel portion, a fuel inlet being located on the insert channelportion.
 80. A mixing apparatus as claimed in claim 79, wherein: thefuel inlet is located in a portion of the insert channel portion havinga curved cross section.
 81. A mixing apparatus as claimed in claim 79,wherein: the insert channel portion comprises a plug attached to one endof the main channel portion, the plug being removable from the body. 82.A mixing apparatus as claimed in claim 79, wherein: the insert channelportion comprises a pre-calibrated insert of the mixing channel.
 83. Amixing apparatus as claimed in claim 79, wherein: the mixing channel hasa curved cross section upstream portion thereof and a transition portionmerging to an exit portion with a rectangular cross section, theupstream portion of the mixing channel being tilted relative to the exitportion thereof.
 84. A mixing apparatus as claimed in claim 79, wherein:the insert channel portion comprises a plug having several primaryinlets spaced therearound.
 85. A mixing apparatus as claimed in claim79, wherein: the mixing channel has a bell-mouth entrance.
 86. A mixingapparatus for mixing fuel and air for combustion in a gas turbine, themixing apparatus comprising: a body having a mixing channel for mixingair and fuel, the mixing channel in one portion thereof having an atleast partly curved cross section.
 87. A mixing apparatus as claimed inclaim 86, wherein: the one portion of the mixing channel has an ellipticcross section.
 88. A mixing apparatus for mixing fuel and air forcombustion in a gas turbine, the mixing apparatus comprising: a bodyhaving a mixing channel for mixing air and fuel, the mixing channelhaving a fuel inlet section which has a plurality of fuel inlets spacedaround a periphery thereof.
 89. A mixing apparatus as claimed in claim79, wherein: the mixing channel has a height/width aspect ratio≦2. 90.An apparatus as claimed claim 86, wherein: the body includes a pluralityof said mixing channels, the mixing channels being regularly spacedabout a dominant axis of the body.
 91. An apparatus as claimed claim 86,wherein: each mixing channel comprises a bore formed in the body of theapparatus.
 92. An apparatus as claimed in claim 86, wherein: a pluralityof primary fuel inlets are provided.
 93. An apparatus as claimed inclaim 86, wherein: a plurality of secondary fuel inlets are provided.94. An apparatus as claimed in claim 93, wherein: the secondary fuelinlets have a shield for providing shielded pilot fuel injection.
 95. Anapparatus as claimed in claim 94, wherein: a configuration of the shieldconforms to an outflow direction of a mixing channel.
 96. An apparatusas claimed in claim 94, wherein: the shield comprises a circular platefor providing shielded flow in a radially inward direction from underthe side plate.
 97. An apparatus as claimed in claim 96, wherein: theplate includes at least one hole therethrough enabling pilot fuel toflow in an axial direction through said plate.
 98. An apparatus asclaimed in claim 86, wherein: the body has a back plate and each mixingchannel is formed in a portion of the body upstanding from the backplate on a fuel side thereof.
 99. An apparatus as claimed in claim 93,wherein: the secondary fuel inlets are adapted to admit fuel at alocation outside the mixing channel part.
 100. An apparatus as claimedin claim 93, wherein: the secondary fuel inlets are adapted to admitfuel into a zone of separated flow on the body.
 101. A radial flowswirler for mixing air and fuel for combustion, the swirler comprising:a body having a primary fuel inlet and a secondary fuel inlet, thesecondary fuel inlet being configured for direct injection of pilotfuel.
 102. A swirler for mixing air and fuel for combustion, the swirlercomprising: a body having a series of mixing channels, the mixingchannels having a bell-mouthed entrance, the mixing channels beinggenerally co-planar with one another and radially inwardly angled inorder to induce swirl.
 103. A swirler as claimed in claim 102, furtherincluding: a backplate adjacent the mixing channels.
 104. A swirler asclaimed in claim 102, wherein: each channel is an elliptical bore. 105.A combustor for burning fuel and air in a gas turbine engine, thecombustor incorporating the mixing apparatus as claimed in claim 79.106. A combustor as claimed in claim 105, wherein: the combustor has acylindrical outer casing wall with an end plate, the mixing apparatusbeing located centrally on the end plate.
 107. A method of calibrating afuel mixer for mixing fuel and air in a gas turbine, the methodcomprising: providing a fuel/air mixing channel having a fuel inletdevice formed with a fuel inlet; calibrating the fuel inlet device; andthen installing the fuel inlet device on to the mixer.
 108. A method asclaimed in claim 107, wherein: calibrating the fuel inlet deviceincludes calibrating the device with respect to fuel flowcharacteristics thereof.
 109. An apparatus as claimed in claim 86,wherein: one or more secondary fuel inlets are provided shielded by anannular ring coaxial with a central axis of the apparatus dischargingpilot fuel in a radially inward direction onto a back wall of theapparatus.
 110. A swirler as claimed in claim 102, wherein: the mixingchannel leads to a toroidal chamber.
 111. A swirler for mixing fuel andair for combusting, the swirler comprising: a body having at least onemixing channel, wherein the mixing channel leads to a toroidal chamber.112. A swirler as claimed in claim 111, wherein: the toroidal chamberhas a same height as a height of the mixing channel.